RubyTk is an interface of the RubyLanguage with TclTk.
For more information see http://wiki.rubygarden.org/Ruby/page/show/RubyTk (This link is not what it was - more a marketing excercise - see RubyGarden.)
A version of the previous page is at http://web.archive.org/web/20070807212956/http://wiki.rubygarden.org/Ruby/page/show/RubyTk
There is a guide at http://www.tutorialspoint.com/ruby/ruby_tk_guide.htm with some example codes.
See RubyGui for other ways to do this.